ek 
BEHAVIOUR OF MATERIALS IN TESTING MACHINE 171 
but they did not remain so, as the load was increased beyond 
_ the elastic limit, and in each case the actual area of the cross section is 
taken as the area of the smallest section. The original and final outlines 
of the mild steel specimen tested in tension are shown in Fig. 241. The 
change in the form of, the wrought-iron specimen in compression as the 
load is increased is clearly shown in Fig. 243. 
Referring to Fig. 242, it will be observed that the maximum nominal 
Stress t square inch. 
20 eT 80 
deel Ee t) 100 
aii Ft 
Bi dood=20 tsi _ 520} PR 
S| foad=30 tone. $40) 4. | Se 
3 Load =30 tone. _ 844 ALT See 
COMPRESSION TEST 
WROUGHT— IRON. 
Sy 
we a 
stress is greater than the nominal stress at fracture, but the actual stress 
at fracture is much greater than the maximum nominal stress. 
In reports on tensile tests for commercial purposes, the maximum 
‘nominal stress is sometimes called the ultimate or breaking stress ; this 
however is wrong, and it should either be called the maximum nominal 
stress, or the maximum stress on the original area. When the term 
maximum stress is used, maximum nominal stress is generally under- 
stood. 
There is no definite ultimate crushing stress for ductile materials, 
‘such as mild steel and wrought-iron, but it will be seen from Fig. 243 
that the curve for the actual stress gets more nearly parallel to the strain 
_ axis as the load is increased. 
‘162. Relation of Elongation to Dimensions of Test Piece.—A study 
of the following results of a test of a specimen of mild steel will lead to 
Fig. 243. 
_ Some important conclusions. Before testing, the specimen was marked off 
carefully in half-inch lengths, and after being broken in tension the two 
: Ne were put together, and the lengths given below the specimen in 
. 244 were measured. The shortest of these lengths, 1°58 inches, is 
